Imagine your new, state-of-the-art solar farm has been operating for a year or two. The performance is solid, but it isn’t quite meeting the ambitious financial models you built. Everything seems fine on the surface, but a hidden thief is quietly stealing 2-3% of your energy yield, and you can’t see it.
This isn’t a hypothetical scenario. It’s the real-world impact of a complex degradation mechanism known as LeTID—Light and elevated Temperature Induced Degradation. For anyone involved in developing, manufacturing, or investing in modern PERC and TOPCon solar modules, understanding this phenomenon is no longer optional—it’s critical.
What is LeTID, and Why Should You Care?
LeTID is a type of power loss that affects certain high-efficiency solar cell technologies, most notably PERC (Passivated Emitter and Rear Cell) and, as the industry is discovering, some TOPCon (Tunnel Oxide Passivated Contact) designs. Unlike initial Light Induced Degradation (LID), which occurs within the first few hours or days of sun exposure, LeTID is a slower, more insidious process. It emerges over months or even years of field operation, triggered by the combined effect of sunlight (carrier injection) and high operating temperatures, typically above 50°C.
The core problem? LeTID can cause significant power loss—sometimes exceeding 5%—that standard performance warranties don’t account for. This creates a major gap between expected and actual energy yield, directly impacting a project’s profitability and return on investment.
But the story of LeTID has a strange twist: after degrading, many modules begin to „heal“ themselves in a process called regeneration. This unique lifecycle makes predicting its long-term impact incredibly difficult without specific, controlled testing.
The LeTID Lifecycle: A Tale of Degradation and Regeneration
LeTID doesn’t cause a simple, linear decline. Instead, it follows a characteristic curve of degradation followed by regeneration, a cycle that can take thousands of hours to play out in the field.
[Image 1: A graph showing the characteristic power loss and subsequent regeneration curve of LeTID over time.]
-
The Degradation Phase: When a susceptible module is exposed to light and elevated temperatures, its maximum power (Pmax) begins to drop. This decline can continue for hundreds or thousands of hours, reaching a point of maximum power loss. For a solar asset owner, this is the period of highest financial risk.
-
The Regeneration Phase: After reaching its lowest point, the module often begins to recover power. This regeneration can be slow, and it doesn’t always restore the module to its initial performance level.
The „aha moment“ for many is realizing that the uncertainty of this curve is the real enemy. How deep will the power drop? How long will it take to recover? Will it recover completely? Answering these questions is impossible by just looking at a datasheet. It requires a specific, standardized test to model the behavior and quantify the risk.
How to Test for LeTID: A Look Inside the IEC TS 63342 Protocol
To address this uncertainty, the industry has developed a standardized testing procedure outlined in the draft technical specification IEC TS 63342. This protocol allows labs to accelerate and precisely map the full LeTID lifecycle, compressing years of field exposure into a matter of weeks.
Here’s how a professional test lab approaches it, transforming abstract risk into concrete data.
Step 1: Establish a Stable Baseline
Before the stress test begins, the module must be stabilized to rule out other degradation effects like LID. This typically involves a defined amount of light soaking (e.g., 5 kWh/m²). Then, a precise initial performance measurement is taken, including its power output (IV curve) and an Electroluminescence (EL) image to document the module’s starting health.
Step 2: The Accelerated Stress Test
The module is then placed inside a specialized climate chamber where the „Light and elevated Temperature“ conditions are simulated.
[Image 2: A diagram or photo of a climate chamber used for LeTID testing, showing modules inside under controlled conditions.]
According to the IEC standard, the test is conducted under very specific conditions:
- Temperature: A constant 75°C (± 2°C) to accelerate the degradation mechanisms.
- Current Injection: A controlled electrical current is passed through the module to simulate the effect of sunlight (typically equivalent to 1 to 10 suns).
These controlled conditions are crucial for producing repeatable, reliable results that can be compared across different module types and manufacturers.
Step 3: Track the Degradation and Regeneration
The test isn’t a single event; it’s a process of continuous monitoring. Every 50-100 hours, the module is cooled to standard test conditions (25°C) and its performance is measured again. This meticulous process maps out the entire degradation and regeneration curve, point by point.
Advanced tools like Electroluminescence imaging are used alongside power measurements. EL images can reveal exactly which cells are degrading and how uniformly the effect is spread across the module, providing invaluable diagnostic information.
[Image 3: A side-by-side comparison of two Electroluminescence (EL) images, one before and one after LeTID, showing the impact on the cells.]
The test continues until the module is considered stable, meaning its power changes by less than 0.2% over the last 100-200 hours of testing. This ensures both the point of maximum degradation and the final regenerated state are captured.
As PV Process Specialist Patrick Thoma notes, „The goal of the test isn’t just to see if degradation occurs, but to precisely map its entire lifecycle to quantify the risk. Only then can you accurately model its impact on long-term energy production.“
It’s Not Just the Cell: Why Your Bill of Materials (BOM) Matters
A common misconception is that LeTID susceptibility is determined by the solar cell alone. In reality, it’s a system-level issue. The entire Bill of Materials (BOM)—from the cell to the encapsulant (like EVA or POE) and even the backsheet—interacts in complex ways under heat and electrical load.
This is why two modules using the exact same PERC cells can exhibit vastly different LeTID behavior. The chemical interactions and physical stresses introduced during lamination play a significant role. Effective risk mitigation therefore requires comprehensive material testing to understand how different components work together.
For companies on the cutting edge, this means any new module design or material combination must be validated. Rigorous solar module prototyping under real industrial conditions is the only way to be certain that a design is robust against LeTID. Ultimately, fine-tuning the manufacturing steps through detailed process optimization can also help mitigate some of the underlying causes of this degradation.
From Lab Data to Financial Certainty
The data from a comprehensive LeTID test—the maximum power loss, the time to degradation, and the final regenerated power level—is more than a technical curiosity. It’s a critical input for creating accurate energy yield models.
For developers and investors, this data de-risks new technology adoption. It allows them to adjust performance forecasts to reflect reality, ensuring financial projections are built on a solid foundation. For manufacturers, it provides the feedback needed to build more resilient and reliable products. By understanding and quantifying LeTID, we turn an unknown liability into a manageable variable.
LeTID Testing FAQ: Your Questions Answered
What’s the difference between LID and LeTID?
LID (Light Induced Degradation) primarily affects p-type silicon and happens quickly, stabilizing within the first few hours or days of sun exposure. LeTID is a much slower process that occurs over hundreds or thousands of hours at elevated temperatures and is characterized by its unique degradation-regeneration cycle.
Does LeTID affect all solar panels?
No. It primarily impacts modern high-efficiency cell architectures like p-type PERC and some n-type TOPCon modules. Older, standard Al-BSF (Aluminum Back Surface Field) cells are generally not susceptible.
Can’t I just trust the manufacturer’s datasheet?
Datasheets are a valuable starting point, but they rarely contain specific LeTID test data for the exact BOM combination you are purchasing. Because susceptibility can vary between production batches and material suppliers, independent testing provides essential verification for large-scale projects or new product development.
How long does a full LeTID test take?
Because the process is slow even when accelerated, a complete test to map both the degradation and regeneration phases can take anywhere from 300 to over 1,000 hours of stress testing until the module is stable.
Your Next Step in Module Reliability
LeTID is a complex challenge, but it is a solvable one. The first step is awareness—recognizing that this hidden degradation mechanism can have a real financial impact on solar assets. Whether you are developing the next generation of solar modules or deploying them in the field, understanding the importance of LeTID testing is fundamental to long-term performance and profitability.
By moving from assumption to data-driven validation, the solar industry can build a more reliable and predictable future, one module at a time.